The inactivation of enzymes by ultraviolet light and visible light in the presence of sensitizing dyes is being investigated with flash photolytic and steady irradiation techniques. The recent work on u.v. inactivation of catenases will be extended to modified enzyme systems, including enzymesubstrate and enzyme-inhibitor complexes, to relate site-specific alterations to the activity loss. The detailed mechanism of electron transfer from excited aromatic residues to the aqueous medium and disulfied bridges via intramolecular transfer will be studied in metallo-enzymes and their apoenzymes. The investigation of photodynamic inactivation will be extended from lysozyme to trypsin, RNase, and papain with emphasis on the role of singlet oxygen and dye binding. The photochemical studies based on steady irradiation and the photostationary techniques will be augmented with fluorescence measurements on dye-protein systems. Particular attention will be given to the interelationship of localized alterations and conformational changes in connection with the photochemical inactivating mechanisms.